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The Beater Movement

This type of intermittent film
drive mechanism (also some-
times called a dog movement)
was very popular in the early
days of cinema, especially at
the ‘economy’ end of the mar-
ket, and will be found in just
about every projector de-
signed for home use.

The operation is quite simple.
The diagram at right gives
some idea of the action.

Sprocket “G” is a regular
sprocket and turns at a speed
which will advance the film at
the desired rate (16 frames per
second). Noting that 35 mm
film has four perforations to the frame, and the sprocket has 16 teeth, a full
rotation would advance the film four frames.
The beater arm located on drive “B”, must be geared to rotate at one quarter of
the speed of “D”, i.e. one revolution per frame of film. As “D” is rotating at a
constant speed, when the beater arm on “B” moves in a downwards direction,
it will pull the film through the gate rapidly, as “B” continues its rotation, the arm
moves in upward direction, the film will remain stationary in the gate during this
phase of the action. The shutter is geared and timed to obscure the pull down
movement of the film, re-
sulting in only the station-
ary frame of film being
seen on the screen, the
closed shutter obscuring
the light during the pull-
down action.

The Ernemann beater
mechanism.

Note the gearing to the
beater arm drive.

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